Combustion and its control are essential features to everyday life. Approximately eighty-five percent of the energy used in the United States alone is derived via combustion processes. Combustion of combustible resources is utilized for, among other things, transportation, heat and power. However, with the prevalent occurrences of combustion, one of the major downsides of these processes is environmental pollution. In particular, the major pollutants produced are nitrogen oxides (NOx), carbon monoxide (CO), unburned hydrocarbons (UHC), soot and sulfur dioxides. Emissions of NOx in particular, have exceeded over twenty-five million short tons in preceding years. Such pollutants have raised public concerns.
In response to public concerns, governments have initiated laws regulating the emission of pollutants. As a result, current combustion systems must efficiently convert the fuel energy into thermal energy with low emissions of NOx, CO, UHC, and soot.
To burn, the fuel must first mix with an oxidant such as air. The resulting mixture must then be supplied with sufficient heat and, if possible, free radicals, which are highly reactive chemical species such as H, OH and O, to ignite. Once ignition occurs, combustion is generally completed within a very short time period. After initial ignition, combustion proceeds via an internal feedback process that ignites the incoming reactants by bringing them into contact within the combustor with hot combustion products and, on occasion, with reactive gas pockets produced by previously injected reactants.
To maintain the flame in the combustor, it must be anchored in a region where the velocity of the incoming reactants flow is low. Low velocities, or long residence times, allow the reactants sufficient time to ignite. In the well known Bunsen burner, the flame is anchored near the burner's rim and the required feedback is accomplished by molecular conduction of heat and molecular diffusion of radicals from the flame into the approaching stream of reactants. In gas turbines, the flame anchoring and required feedback are typically accomplished by use of one or more swirlers that create recirculation regions of low velocities for anchoring the flame and back flow of hot combustion products and reacting pockets that ignites the incoming reactants. In ramjets and afterburners, this is accomplished by inserting bluff bodies, such as a V-shaped gutter, into the combustor to generate regions of low flow velocities and recirculation of hot combustion pockets and reacting gas pockets to anchor the flame and ignite the reactants.
More recently, in an effort to reduce NOx emissions in industrial processes, the use of high velocity fuel and air jets to attain what is referred to as flameless combustion has been advocated. U.S. Pat. No. 5,570,679 discloses a flameless combustion system. In the '679 patent, an impulse burner is disclosed. Fuel and air jets that are spatially separated by specified distances are injected into the combustor or process with high velocities. The system incorporates two separate operating states. In the first state, the burner is first switched such that a first fuel valve is opened and a second fuel valve is closed. The fuel and oxidant are mixed in a combustion chamber and ignited with stable flame development and the flame gases emerge through an outlet opening in the combustion chamber to heat up the furnace chamber. As soon as the furnace chamber is heated to the ignition temperature of the fuel, a control unit switches the burner over to a second operating state by closing of the first fuel valve and opening a second fuel valve. In this second operating state, no fuel is introduced into the combustion chamber and as a consequence, the burning of the fuel in a flame in the combustion chamber is essentially suppressed entirely. The fuel is fed into the furnace chamber exclusively.
Because of their high momentum, the incoming fuel and oxidant jets act as pumps entraining large quantities of hot combustion products within the furnace chamber. Since the furnace chamber has been heated up to the ignition temperature of the fuel, the reaction of the fuel with the combustion oxidant takes place in a distributed combustion process along the vessel without a discernible flame. Consequently, this process has been referred to as flameless combustion or flameless oxidation. Since this process requires that the incoming reactants jets mix with large quantities of hot products, its combustion intensity, i.e., amount of fuel burned per unit volume per second, is low. Also, the system requires high flow velocity of the fuel jets to create the pump action necessary for mixing the fuel with the hot combustion products. Additionally, since a significant fraction of the large kinetic energy of the injected reactants jets is dissipated within the furnace, the process experiences large pressure losses. Consequently, in its current design, this process is not suitable for application to land-based gas turbines and aircraft engine's combustors and other processes which require high combustion intensity and/or low pressure losses.
In another combustion system, often referred to as well stirred or jet stirred combustor, fuel and oxidant are mixed upstream of the combustion chamber and the resulting combustible mixture is injected via one or more high velocity jets into a relatively small combustor volume. The high momentum of the incoming jets produces very fast mixing of the incoming reactants with the hot combustion products and burning gases within the combustor, resulting in a very rapid ignition and combustion of the reactants in a combustion process that is nearly uniformly distributed throughout the combustor volume.
Generally, existing combustion systems minimize NOx emissions by keeping the temperatures throughout the combustor volume as low as possible. A maximum target temperature is approximately 1800K, which is the threshold above which thermal NOx starts forming via the Zeldovich mechanism. Another requirement for minimizing NOx formation is that the residence time of the reacting species and combustion products in high temperature regions, where NOx is readily formed, be minimized. On the other hand, temperatures and the residence times of the reacting gases and hot combustion products inside these combustors must be high enough to completely burn the fuel and keep the emissions of CO, UHC, and soot below government limits.
Gas turbine systems are known to include a compressor for compressing air; a combustor for producing a hot gas by reacting the fuel with the compressed air provided by the compressor, and a turbine for expanding the hot gas to extract shaft power. The combustion process in many older gas turbine engines is dominated by diffusion flames burning at or near stoichiometric conditions with flame temperatures exceeding 3,000 degrees F. Past the combustion zone and prior to the turbine inlet the hot gases are diluted by extra “cold” air from the compressor discharge to limit the turbine inlet temperature to a permissible level. Such combustion will produce a high level of oxides of nitrogen (NOx). Current emissions regulations have greatly reduced the allowable levels of NOx emissions. Lean premixed combustion has been developed to reduce the peak flame temperatures and to correspondingly reduce the production of NOx in gas turbine engines. In a premixed combustion process, fuel and air are premixed in a premixing section upstream of the combustor. The fuel-air mixture is then introduced into a combustion chamber where it is burned. U.S. Pat. No. 6,082,111 describes a gas turbine engine utilizing a can annular premix combustor design. Multiple premixers are positioned in a ring to provide a premixed fuel/air mixture to a combustion chamber. A pilot fuel nozzle is located at the center of the ring to provide a flow of pilot fuel to the combustion chamber.
The design of a gas turbine combustor is complicated by the necessity for the gas turbine engine to operate reliably with a low level of emissions at a variety of power levels. High power operation tends to increase the generation of oxides of nitrogen. Low power operation at lower combustion temperatures tends to increase the generation of carbon monoxide and unburned hydrocarbons due to incomplete combustion of the fuel. Under all operating conditions, it is important to ensure the stability of the flame to avoid unexpected flameout, damaging levels of acoustic vibrations, and damaging flashback of the flame from the combustion chamber into the fuel premix section upstream of the combustor. A relatively rich fuel/air mixture will improve the stability of the combustion process but will have an adverse affect on the level of emissions. A careful balance must be achieved among these various constraints in order to provide a reliable machine capable of satisfying very strict contemporary and future emissions regulations.
With respect to gas turbines, FIG. 9 illustrates a schematic diagram of a typical gas turbine system 80. A compressor 82 draws in ambient air 84 and delivers compressed air 86 to a combustor 88. A fuel supply 90 delivers fuel 92 to combustor 88 where it reacts with the compressed air to produce high temperature combustion gas 94. The combustion gas 94 is expanded through a turbine 96 to produce shaft horsepower driving shaft 95 for driving compressor 82 and a load such as an electrical generator 98. Gas turbines having an annular combustion chamber exist including a plurality of burners disposed in one or more concentric rings for providing fuel into a single toroidal annulus. U.S. Pat. No. 5,400,587 describes one such annular combustion chamber design.
With respect to gas turbines for jet engines, FIG. 10 illustrates a prior art LM6000 engine commercially available from General Electric Aircraft Engines, Cincinnati, Ohio. Gas turbine engine 100 includes a low pressure compressor 102, a high pressure compressor 104, and a combustor 106. Engine 100 also includes a high pressure turbine 108 and a low pressure turbine 110. Compressor 102 and turbine 110 are coupled by a first shaft 112, and compressor 104 and turbine 108 are coupled by a second shaft 114. Engine 100 also includes a center longitudinal axis of symmetry 116 extending there through.
For jet engine design, there are historically three types of combustion chambers. There are multiple chambers, the turbo-annular chamber, and the annular chamber. These designs utilize a combustion chamber which has an inlet for receiving compressed air in the proximity of the compressor and a gas discharge at the opposite end in the proximity of the turbine. In operation, air flows through the low pressure compressor and compressed air is supplied from the low pressure compressor to the high pressure compressor. The highly compressed air is delivered to the combustor on the compressor side of the system. Gas flow from the combustor drives the turbines and exits the gas turbine engine through a nozzle.
As gas turbines and jet engines employ combustion systems, there is a need to develop a simple combustion system which produces low NOx emissions while being used in gas turbines and jet engine systems. In addition to gas turbine generators and jet engines, combustors are also utilized for industrial boilers to assist in generating steam to produce electricity and the like. Also, combustors are utilized in domestic and industrial heating processes such as water and air heating and material drying.
A primary problem with most combustion systems as mentioned above is the generation of pollutants such as NOx among others during the combustion of the fuel and air. This results because of the stoichiometry of the reacting fuel and oxidant streams. The stoichiometric quantity of an oxidizer is just that amount needed to completely burn the quantity of fuel. If more than a stoichiometric quantity of oxidizer is supplied, the mixture is said to be fuel lean, while supplying less than the stoichiometric oxidizer results in a fuel-rich mixture. The equivalence ratio is commonly used to indicate if the mixture is rich or lean. Typically to produce low NOx, the combustion is run fuel-lean. This requires a larger quantity of oxidant to be present and typically the utilization of swirlers to mix the fuel and the air prior to combustion. A typical combustion process is configured along an axis with the oxidant and fuel mixed upstream of a flame with combustion products exiting the combustor downstream from the flame. While suitable for their intended purposes, such systems utilize complicated structures to mix the air and fuel and are not always effective in their mixing. Furthermore, reducing the oxidants generally results in higher combustion process temperatures which produce higher NOx emissions.
The object of the invention is to create a simple and low cost combustion system that uses its geometrical configuration to attain complete combustion of fuels over a wide range of fuel flow rates, while generating low emissions of NOx, CO, UHC and soot.
Another object of the invented combustion system is to provide means for complete combustion of gaseous and liquid fuels when burned in premixed and non-premixed modes of combustion with comparable low emissions of NOx, CO, UHC and soot.
Another object of this invention is to provide capabilities for producing a robust combustion process that does not excite detrimental combustion instabilities in the combustion system when it burns fuels in premixed and non-premixed modes of combustion.
Another object of this invention is to use the geometrical arrangement of the combustion system to establish the feedback between incoming reactants and out flowing hot combustion products that ignites the reactants over a wide range of fuel flow rates while keeping emissions of NOx, CO, UHC and soot below mandated government limits.